Today's hybrid vehicles are usually equipped with a combustion drive and an electric drive, which, as shown in FIG. 4, is supplied with a voltage of 300 V for example by a high-voltage battery 401. High-voltage battery 401 is connected via a switch system 403 to an inverter 405, which generates a three-phase alternating voltage for an electric motor 407. Switch module 403 is switched in for example by a main protector 409 prior to starting electric motor 407. Main protector 409 is coupled for example mechanically with the aid of a mechanical coupling 411 to switch module 403 and draws the energy for actuating the latter from a low-voltage battery 413 having a 12 V direct voltage. Furthermore, control terminals 415 are provided, which are connected for example to a vehicle start button such that a starting process of the vehicle also initiates the closing of switch module 403.
If low-voltage battery 413 is discharged, however, then switch module 403 cannot be closed such that high-voltage battery 401 cannot be connected to inverter 405 and the electric motor consequently cannot be started. Moreover, in spite of the existing high-voltage battery 401, the vehicle electrical system supply of 12 V direct voltage is not ensured in the event of a failure of low-voltage battery 413 such that in addition to the electric motor other vehicle systems also cannot be supplied with electrical energy.
PCT International Published Patent Application No. WO 2007/115919 describes a hybrid drive, a vehicle electrical system battery being used to start an electric motor when the high-voltage battery is discharged. If the vehicle electrical system battery is discharged as well, then the starting procedure is performed with the aid of a jumper cable and a donor battery.